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The Pathogen Path: Scientist tracks how bacteria hitch ride on plants to get to humans

To dine with Jeri Barak is to take a lesson in applied food safety. Barak, an assistant professor of plant pathology, begins by sorting through her salad, plucking out any greens that appear dark and wet and moving them to another plate. It’s not the appearance that bothers her—it’s the small, but deadly, chance that those leaves host colonies of Salmonella enterica.

Why would salmonella, bacteria that thrive in the warm-blooded environs of an animal, hang out in a pile of lettuce? That’s what Barak would like to know. For the past 10 years, she’s been studying how the bacteria use plants as a mode of transportation to arrive at a more favorable destination.

Given the choice between alighting upon a plant or an animal, salmonella cells would pick an animal every time, Barak explains. Animals provide just the right milieu for the bacteria to grow and reproduce. By comparison, plants are inhospitable wastelands. But Barak has found that when salmonella cells wind up on a tomato or cauliflower plant, they are capable of hunkering down and waiting for something better to come along. “They want to get to an animal host, so why not get onto the food that your host eats?” Barak says. “It’s a smart strategy.”

That strategy is abetted by Americans buying more fresh produce. In the past, most families boiled vegetables like spinach, helping kill off pathogens. But as more veggies are eaten raw, foodborne illnesses from contaminated produce have increased significantly. Over the past 40 years, the incidence of produce-related outbreaks has grown from less than 1 percent to more than 12 percent of reported cases. “There is even some evidence that the number of salmonellosis outbreaks caused by people eating produce is now higher than those caused by eating eggs, chicken and other animal products,” says Barak.

Barak was among the first handful of researchers to start studying human pathogens in the context of plant systems. She was just launching her research career in 1996, when a major E. coli outbreak in Japan caused 17 deaths and more than 6,000 illnesses. The source in that case was sprouts, and it motivated the U.S. government to fund research on pathogens in produce.

In the lab, Barak has been working to identify the genes that enable salmonella to hang on for the ride, with the long-term goal of using this knowledge to improve food safety. So far she has pinpointed more than a dozen key genes involved in attaching and adhering to plants. She is also exploring a number of important extrinsic factors. She discovered, for instance, that salmonella thrives when tomato plants are infected with Xanthamonas vesicatoria, a common plant pathogen. “During disease,” she explains, “there are nutrients leaking out, so there’s a lot of stuff for salmonella to eat and everything just grows.” On the positive side, Barak has found a number of heirloom tomato varieties that salmonella can’t attach to. She is in the process of figuring out what makes these plants impervious to the bacterium, which could help speed the breeding of salmonella-free tomato varieties down the line.

In the meantime, Barak has no plans to swear off her vegetarian diet and hopes no one else will either. “That would be the worst thing that could happen,” she says. Just watch out for the slimy lettuce.